The value of the voltage supplied by a lambda sensor depends not only on the lambda value at the location of the sensor, but also on its temperature. The dependence is particularly strong in the rich branch. Due to the temperature response, the measured results are falsified to such an extent that, below a certain temperature, the voltages supplied by the sensor cannot be utilized at all for control. The internal resistance of the sensor, for example, is measured, which decreases with increasing temperature, in order to detect the cut-in threshold from which the values are used.
A method for internal resistance measurement is described in U.S. Pat. No. 4,742,808. The sensor voltage is measured once in the unloaded state and then under load by a load resistor with a predetermined resistance value. From the two voltages and the known resistance value, the actual internal resistance is calculated. This is compared with a threshold value, and then, if the actual value is below the threshold value, the voltage values are used for lambda control.
A simpler method, in which the temperature dependence of the internal resistance is utilized, is known from U.S. Pat. No. 4,528,957, but this method does not make it necessary to work out specifically the internal resistance. The known method uses the fact that the voltage picked off at a load resistor lying in series with the sensor does not only change whenever the sensor voltage changes due to lambda value fluctuations, but that the voltage also changes whenever the internal resistance changes. It is therefore not necessary at all to work out the internal resistance with the aid of the load resistor; instead, it is sufficient to compare the voltage picked off at the load resistor with a voltage threshold value in order to detect the sensor readiness. Actually, two thresholds are necessary, since two different voltages can be supplied with the same internal resistance, dependent on whether the sensor is just then supplied with exhaust gas which derives from the rich mixture or from the lean mixture.
In the case of the above-mentioned methods for sensor-readiness detection, the threshold values are set such that control readiness of the sensor is only detected when the internal resistance is only changing slightly and consequently hardly falsifies the measured result any longer.
An arrangement is known from EP-A3-0 152 293 which considers the internal resistance of a limit current sensor when determining a lambda value from a current flowing through the limit current sensor.
The invention is based on the object of specifying a method for lambda value detection by means of a lambda sensor, which supplies a voltage dependent upon the oxygen content to be measured, which method operates reliably even at lower temperatures than before. The invention is furthermore based on the object of specifying uses for this method. Finally, the invention is based on the object of providing an apparatus for carrying out such a method.